A video signal typically includes vertical display intervals that comprise a plurality of horizontal line intervals, e.g. 525 lines per vertical interval in NTSC video systems. A portion of each vertical interval is usually designated as a vertical blanking interval. The vertical blanking interval may span a plurality of horizontal line intervals, e.g. in excess of 20 horizontal line intervals. The beginning of each vertical and horizontal interval is identified by respective vertical and horizontal sync pulses that are included in a composite video signal.
The content of the video signal during blanking intervals is usually not intended for display as part of the normal video image. The lack of image information in blanking intervals makes it possible to insert auxiliary information, e.g. teletext and closed caption data, into blanking intervals. The standards for each type of auxiliary information specify the positioning of the information within a vertical blanking interval. For example, the present closed captioning standard (see e.g. 47 CFR .sctn..sctn.15.119 and 73.682) specifies that digital data corresponding to ASCII characters for closed captioning must be in line 21 of vertical blanking.
An approach to recovery of auxiliary information is to accurately identify a specific line interval, e.g. line 21, containing auxiliary information during a vertical blanking interval by counting horizontal sync pulses. For example, a horizontal line counter could be initialized by a vertical sync pulse and clocked by horizontal sync pulses. Ideally, the count value would then represent the line number.
A video receiver may produce various versions of horizontal and vertical sync signals that might be used to control a line counter. For example, a sync separator may produce sync signals directly from the received composite video signal. FCC requirements specify precise timing limits for composite video signals. These timing limits might suggest that the output of a sync separator would be a reliable source of control signals for a line counter. However, weak signals combined with the occurrence of equalization pulses during vertical blanking may produce an irregular horizontal sync pulse waveform at the sync separator output that is not a reliable source of clock pulses for a line counter. Thus, it may be desirable to use a more regular horizontal sync waveform to clock a line counter. For example, the output of a horizontal phase-locked-loop (PLL) that is typically included in a video receiver for deflection purposes produces a regular pulse waveform at the horizontal line rate.
As a result of FCC specifications, the start of data within any horizontal line is accurately timed with respect to the composite video or sync separator output, but not necessarily with respect to the horizontal PLL output. Minor timing shifts do not adversely affect the display of normal video images. However, significant timing errors may be introduced if the operation of the horizontal PLL is disturbed. For example, switching between the dual video read heads in a video cassette recorder (VCR) may produce an abnormal horizontal line period that differs significantly from the nominal 64 .mu.s period. The deviation in the line period may produce a perturbation in the horizontal PLL that is manifested as a substantial phase shift between the composite sync signal at the sync separator output and the horizontal pulse waveform at the horizontal PLL output. The locking action of the PLL gradually corrects the perturbation such that the phase error is substantially eliminated before visible display begins. A significant phase shift may exist, however, for many of the line periods within vertical blanking. Thus, the actual beginning of the information in a horizontal line during vertical blanking as indicated by the sync separator output may differ in time from the beginning of the same line as indicated by the output of a horizontal PLL.
The residual phase shift that remains during line intervals within vertical blanking may preclude retrieving auxiliary video data that occurs during line periods within the vertical blanking interval. For example, consider a closed caption decoder that identifies the beginning of closed caption data in line 21 based on the count value from a line counter that is clocked by the output of a horizontal PLL. If the video signal containing closed caption data originates in a VCR, the horizontal PLL error caused by read head switching may cause the line counter to exhibit the value 21 at a time that is not synchronized to the beginning of data in line 21. Thus, the closed caption decoder would be attempting to extract closed caption data from line 21 either before or after line 21 actually begins. Corrupted closed caption data may be produced.